Fusarium sporotrichioides mutant strain capable of producing both dideacetylcalonectrin and deacetylcalonectrin

ABSTRACT

Mutants blocked or altered in the production of the trichothecene mycotoxin T-2 have been selected following UV mutagenesis of Fusarium sporotrichioides NRRL 3299. One mutant, NRRL 18339, accumulates the rare trichothecenes and dideacetylaclonectrin and deacetylcalonectrin. The second mutant, NRRL 18340, accumulates trichodiene, the first intermediate on the trichothecene biosynthetic pathway. These compounds are useful toxins or toxic precursors for the production of or investigation of immunotoxins for cancer therapy.

BACKGROUND OF THE INVENTION

Targeting the delivery of toxins to malignant cells by complexing thetoxin to monoclonal antibodies which recognize the cancer cells isbecoming an effective approach to cancer therapy. The toxin-monoclonalantibody conjugates preferentially bind to and selectively kill thecancer cells [D.A. Vallera et al., Science 222: 512 (1983); E.S.Vietetta et al., Science 238; 1098 (1987)].

As a group, the trichothecenes appear to be promising toxins for theproduction of immunotoxins. The molecules are relatively small in size,and the class includes members with a wide range of toxic properties.Trichothecenes are potent inhibitors of protein synthesis [C.J. Carterand M. Cannon, Biochem. J. 166: 399-409 (1977); see also: ProtectionAgainst Trichothecene Mycotoxins, Natural Academy Press, Washington, DCpp. 129-138 (1983)]. This diversity provides the potential to designanticancer agents to meet specific requirements. There is a medicallyimportant need for a variety of trichothecenes.

Trichodiene was isolated from Trichothecium roseum [S. Nozoe and Y.Machida, Tetrahedron 28: 5105-5111 (1972)]and is the first intermediatein the trichothecene biosynthetic pathway. Dideacetylcalonectrin anddeacetylcalonectrin are rare trichothecenes; these materials haveheretofore been available only from complex synthetic methods or asminor components in fermentation broths [R. Greenhalgh et al., J. Agric.Food Chem. 34: 98-102 1986)]. It has not been previously economicallyfeasible to produce these compounds in large quantity.

SUMMARY OF THE INVENTION

We have now discovered novel mutants selected following UV mutagenesisof Fusarium sporotrichioides NRRL 3299 which accumulate trichothecenesand trichodiene.

Mutant NRRL 18339 produces the trichothecenes dideacetylcalonectrin anddeacetylcalonectrin; mutant NRRL 18340 accumulates trichodiene, thefirst intermediate on the trichothecene biosynthetic pathway.

In accordance with this discovery, it is an object of the invention toproduce relatively high yields of dideacetylcalonectrin by acommercially feasible fermentation process.

A further object of the invention is to produce relatively high yieldsof deacetylcalonectrin by a commercially feasible fermentation process.

A further object of the invention is to produce relatively high yieldsof trichodiene by a commercially feasible fermentation process.

A further object of the invention is to define two novel mutant speciesof Fusarium which are capable of producing trichothecenes andtrichodiene.

Other objects and advantages of this invention will become readilyapparent from the ensuring description.

DETAILED DESCRIPTION OF THE INVENTION

The fungal strains used in this invention were derived by UV mutagenesisof Fusarium sporotrichioides NRRL 3299 (ATCC 24043) deposited with theARS Culture Collection, Peoria, IL. This strain was originally isolatedfrom corn in France.

Microconidia are obtained for mutagenesis as follows. Frozen glycerolstocks of conidia are inoculated, after thawing, on agar plats, whichare then incubated for several days. This results in cultures whichproduce primarily one- and two-celled microconidia. The microconidiathus obtained are then exposed to UV light and incubated in the dark forseveral days. Surviving colonies are then screened for T-2 productionusing the monoclonal antibody technique developed by Hunter et al. andmodified by Beremand [K.W. Hunter et al., Appl. Eviron, Microbiol. 49:168-172 (1985), and M. N. Beremand, Appl. Environ. Microbiol. 53:1855-1859 (1987)]. Mutants which are blocked or altered in thebiosynthesis of T-2 toxin are selected and further examined for theproduction of other trichothecene compounds.

Using the above technique, two novel mutant species were discoveredwhich produce relatively large quantities of rare trichothecenes andrelated materials. Mutant strain NRRL 18339 produces15-deacetylcalonectrin (Compound 1) and 3,15-dideacetylcalonectrin(Compound 2). Strain NRRL 18340 accumulates trichodiene (Compound 3) anddoes not produce any 12,13-epoxytrichothecenes. These two mutant speciesare deposited with the ARS Culture Collection, Peoria, IL.

To prepare quantities of the heretofore rare trichothecene derivatives,conidial suspensions are prepared from 1- to 2-week-old cultures grownon solid media.

A growth medium comprising glucose, 5-10%, preferably 5%; yeast,0.1-0.2%, preferably 0.1%; peptone, 0.1-0.2%, preferably 0.1%, isprepared and inoculated with conidia to a level of 10³ -10⁵, preferably5×10⁴ per ml. The cultures are incubated from 5 to 14 days, preferably14 days, at 25-30° C., preferably 28° C.

Analyses indicate 100 to 1000 μM/L of products are produced under theabove conditions.

The trichothecenes and trichodiene may be isolated by standardtechniques such as solvent extraction of the culture, including bothcellular material and growth media, and subsequent purification bychromatographic procedures.

Analysis of samples by gas chromatography-mass spectrometry (GC/MS)verified that the compound produced by mutant NRRL 18340 wastrichodiene.

Small quantities of compounds 1 and 2 were isolated by preparative thinlayer chromatography from a 7-day-old 25 ml liquid shake culture ofmutant NRRL 18339. Analysis of these compounds by GC/MS demonstratedthat both compounds contained the 12,13-epoxy-trichothec-9-ene nucleusand thus confirmed that they were trichothecenes. The chemicalionization mass spectra of trimethylsilyl-derivatized and underivatizedsamples further revealed that compound 1 contained a free hydroxyl groupand an acetate, while compound 2 contained 2 hydroxyl groups.

The structures of compounds 1 and 2 were determined by NMR analysis oflarger amounts of these compounds which were purified from the culturefiltrate of a 4 L liquid fermentation of strain NRRL 18339. Based on the¹ H and ¹³ C-NMR spectra, compounds 1 and 2 were identified as15-deacetylcalonectrin (DECAL) and 3,19-dideacetylcalonectrin (DIDECAL).Both compounds contain 2 less oxygen moieties than found in T-2 toxin.

Finally, GC and GC/MS analyses of crude ethyl acetate extracts of NRRL18339 revealed that this mutant produces small amounts of T-2 toxin.Thus NRRL 18339 appears to be a slightly leaky mutant.

Liquid cultures were left intact or separated by filtration into myceliaand filtrate fractions prior to extraction with ethyl acetate. GCanalysis of these samples demonstrated that all of the T-2 toxinproduced by the wild type parent was excreted into the medium (Table 1).In contrast, 100% of the trichodiene produced by NRRL 18340 remainedassociated with the mycelia. The calonectrin analogues produced by NRRL18339 displayed an intermediate response; 60 to 70% of these twocompounds was isolated from the culture filtrate, and the remaining 30to 40% was isolated from the mycelial fraction.

Measurement of radial colony growth rate on V8-juice agar and M-100minimal medium revealed that the mutant strains were prototrophic andthat they retained wild type growth rates. Likewise, growth of themutants in liquid shake cultures, as measured by mycelial dry weights,was indistinguishable from that of the wild type parent. Grossmorphology of the mutant and the wild type cultures were the same forall strains growing on solid and liquid media.

Without desiring to be bound by any theory of operation, it is believedthat the inability of the mutant to produce T-2 toxin is associated withchanges in conidiation. Both mutants produce two- to sixfold fewerconidia than the wild type parent when grown on V-8 juice agar medium.In addition, the nontrichothecene, trichodiene accumulating mutant alsoproduces conidia with an altered morphology. A diagnostic species traitof F. sporotrichioides is the formation of napiform or pear-shapedmicroconidia. Mutant NRRL 18340 fails to make napiform conidia whengrown on V8-juice agar. The inability to make napiform conidia can bereversed when NRRL 18340 is grown in the presence of exogeneouslysupplied T-2 toxin. Furthermore, T-2 toxin appears to be required duringconidiation.

                  TABLE I                                                         ______________________________________                                        Localization of Trichodiene and Trichothecenes Accumulated by                 Wild Type and Mutant Strains of Fusarium sporotrichioides                     Grown in Liquid Shake Cultures.sup.a                                          NRRL 18340     NRRL 18339     NRRL 3299                                              Trichodiene Didecal  15-Decal                                                                              T-2 Toxin                                 Sample (μg/ml)  (μg/ml)                                                                             (μg/ml)                                                                            (μg/ml)                                ______________________________________                                        Whole  126         127      80      265                                       culture                                                                       Mycelia                                                                              122         39       30       0                                        Filtrate                                                                              0          88       50      292                                       ______________________________________                                         .sup.a YEPD5G media was inoculated to a final density of 5-7 ×          10.sup.4 conidia per ml and incubated 7 to 8 days at 28° C. and        180-200 rpm.                                                             

NRRL 18340 conidia formed in the absence of T-2 toxin are not convertedto pear-shaped conidia by subsequent incubation (for up to 24 hrs) withT-2 toxin. Addition of T-2 toxin to the wild type and mutant NRRL 18339also appears to stimulate production of napiform conidia in thesestrains. Addition of T-2 toxin does not, however, restore the level ofconidia production in either NRRL 18340 or NRRL 18339 to that observedfor the wild type parent. The above results provide the first indicationthat trichothecenes may play a role in fungal development.

The following examples are intended only to further illustrate theinvention and are not intended to limit the scope of the invention whichis defined by the claims.

EXAMPLE 1

UV Mutagenesis and Mutant Screen

Microconidia were obtained from V-8 agar plates that were incubated for4 days following inoculation with approximately 2.5×10⁶ freshly thawedconidia from frozen (-70° C.) glycerol stocks. This procedure yieldedprimarily one- and two-celled microconidia, with approximately 65% ofthe total being one-celled. The microconidia were exposed to UV light(254 nm) on agar plates until 90% kill was achieved [see J. Avalos etal., Appl. Environ. Microbiol. 49: 187-191 (1985) and Beremand, supra].

The UV-treated plates were immediately placed in the dark and incubatedfor 2 to 3 days in the growth chamber. To screen for T-2 toxinproduction, a portion of each surviving colony was transferred to a wellin a 96-well microtiter plate (Falcon 3072; Becton Dickinson Labware,Oxnard, Calif.) containing 150 μl of YEPD-5G per well. The plates weresealed with Parafilm to reduce evaporation and were incubated at 28° C.at 50 to 60 rpm on a minishaker (Dynatech Laboratories, Inc.,Alexandria, VA). After 4 to 5 days, the fungal mat was removed from eachwell with a toothpick to yield a cleared supernatant. The supernatantswere stored at -20° C. until the immunoassays were performed. For theimmunoassays, each microtiter plate contained a blank control well(which yielded an A₄₁₀ value for media only), a positive well (whichyielded an A₄₁₀ value for a T-2 toxin standard solution containing2×10⁻⁴ M T-2), and 94 test samples (culture filtrates). Colonies whichproduced test samples that yielded an A₄₁₀ value either equal to that ofthe blank control or significantly higher than that of the T-2 controlwere scored as potential toxin mutants. To screen for auxotrophy,strains were tested for their ability to grow on M-100 minimal medium.

Competitive inhibition enzyme-linked immunoassays (CIEIAs) wereperformed with affinity-purified monoclonal antibody 15H6 as describedby Hunter et al., supra, except that methanol could be omitted from thephosphate-buffered saline-Tween 20 solution without consequence. Afraction of the culture supernatant, the T-2 standard solution, or thecontrol medium (50 μl) was mixed in polystyrene microtiter plates(Falcon 3190) with 50 μl of 15H6 monoclonal antibody diluted to 5 μμg/mlin phosphate-buffered saline containing 0.05% Tween 20. Followingincubation for 1 hr at room temperature, fractions (50 μl) of thesesamples were transferred to the wells of round-bottom polyvinylmicrotiter plates (Dynatech) with T-2-bovine serum albumin (10 μg/ml in0.1 M Tris hydrochloride [pH 8.2]; Sigma Chemical Co., St. Louis, MO) byovernight incubation at 4° C. After 30 min of incubation at roomtemperature, the wells were washed five times with phosphate-bufferedsaline-0.05% Tween 20 and successively incubated with rabbit anti-mousekappa light chain antiserum (ICN Immuno chemicals, Elkhart, IN), goatanti-rabbit immunoglobulin G-alkaline phosphate conjugate (Sigma), andenzyme substrate (p-nitrophenylphosphate; Sigma) as described previously(9). Assays were measured at 410 nm on an automatic microtiter platereader (Dynatech).

EXAMPLE 2 Culture Growth Conditions

Cultures were grown on V-8 agar medium slants or plates on analternating 12-hr 25° C. light/20° C. dark schedule. For long-termstorage, strains were maintained on V-8 agar slants at 4° C. and werestored as conidial suspensions in 10 to 15% (vol/vol) glycerol at -90°C. For all assays, fresh transfers of the strains were obtained fromstock cultures stored at 4° C.

Trichothecene production was measured in liquid shake cultures. Conidialsuspensions were prepared from 1- to 2-week-old cultures grown on V-8juice agar plates. 2.8-Liter Fernback flasks containing 1 L of 5%glucose-0.1% yeast extract-0.1% peptone were inoculated with conidia toa final concentration of 10⁴ per ml. Cultures were incubated at 200 rpmon a gyratory shaker. Incubation was continued as described above for atotal of 7 days, at which time the products were isolated and analyzed.

EXAMPLE 3 Instrumentation for Analysis

Trichothecenes and trichodiene were identified and quantitated by gaschromatography (GC)-mass spectrometry (MS) with a mass spectrometer(TSQ46; Finnegan). Electron ionization (EI) spectra were obtained at 70V. Chemical ionization (CI) spectra were recorded at a measured sourcetemperature of 100° C. Isobutane (0.3 torr) was the reagent gas.Conventional mass spectra were obtained by operating the first twoquadrupoles in the all-pass mode and scanning the third quadrupole. Intandem MS experiments, the protonated molecule (MH⁺) was selected byquadrupole 1, and daughter fragments were formed in quadrupole 2 bycollision with argon (1 mtorr and 20 V). The resulting daughter ionswere mass analyzed by the third quadrupole.

GC/MS analyses were made on either a capillary or a packed column. Afused silica capillary column (30 M by 0.25 mm; DB-1; J&W Scientific,Rancho Cordova, CA) was used. The linear flow rate of helium in thecolumn was 50 cm/s. The injection port temperature was held at 250° C.Samples were injected in the split mode with a split ratio ofapproximately 90:1. The outlet of the capillary column was directlycoupled into the source of the mass spectrometer. The packed column wasa glass column (2m×2 mm) packed with 1% OV-1 on 100-200 mesh ChromosorbW. The flow rate was 20 ml/min, and the column was coupled to the massspectrometer via a glass jet separator.

EXAMPLE 4 Trichothecene Analysis

Trimethylsilyl derivatives were made from 50-μl portions of the culturemedia extracts by evaporation of the ethyl acetate under nitrogen andthe addition of 50 μl of Tri-Sil/TBT (Pierce Chemical Co., Rockford,IL). The samples were held at 50° C. for 1 hr before 1 μl was analyzedby GC/MS. For capillary GC/MS, the starting GC temperature was 180° C.At 2 min after injection, the column was heated at 6° C./min to a finaltemperature of 270° C.; it was then held at this temperature. When usedin the CI mode, the mass spectrometer was scanned from 90 to 700daltons. In the EI mode, the mass spectrometer was scanned from 40 to700 daltons. Repetitive scans of 1 s were acquired, and GC peaks weredetected form the reconstructed ion chromatogram and were identified bycomparison with standards. For a more rapid quantitative method, thepacked column was programmed from 200°-250° C. at 10° C./min. Theprotonated molecular ion and the most intense ion for the compounds ofinterest were monitored in the selected ion mode. Response factors weredetermined from linear regression of the response from injection ofknown amounts of these compounds across the range of 1 ng to 10 μg andwere used to quantitate the amount of individual trichothecenes in thesamples. The total analysis time with the packed column was less than 6min per sample.

EXAMPLE 5

Trichodiene Analysis

The culture extracts were analyzed for trichodiene withoutderivatization. The samples were injected into packed or capillarycolumns at 120° C. and after 5 min the column over temperature wasraised to 250° C. at 4° C./min. Trichodiene was identified in theculture extracts based on comparison the EI and CI mass spectra and thedaughter ion spectrum of the protanated molecule with those for thechemically synthesized and the biosynthetically produced standards [VanMiddlesworth et al., J. Chem. Soc. Chem. Commun. 1986: 1156-1157]. TheEI spectrum (FIG. 1) had a weak molecular ion (m/z 204). The mostabundant fragment at m/a 109 was created by cleavage between the tworings. The intense dimethyl cyclohexadiene fragment at m/z 108 arosefrom the transfer of a hydrogen to the vinyl group and cleavage betweenthe two rings. The isobutane CI spectrum (FIG. 1) of trichodiene has anintense protonated molecule (m/z 205) and two intense fragments at m/z95 and m/z 109 which resulted from cleavage between the two rings. Thedaughter spectrum of the protonated molecule had only two abundantfragments, the m/z 109 and m/z 95 ions. The retention times and massspectra for the trichodiene standard and for trichodiene in theancymidol-treated cultures were identical. Close examination of CIspectra from chromatograms of extracts from ancymidol-treated culturesrevealed a complex pattern of minor sesquiterpenoid components withsignals at m/z 205. However, none of these minor components had intensem/z 109 signals or any detectable daughters of the m/z 205 parent at m/z109 in MS/MS scans. Quantitation of the amount of trichodiene inancymidol-treated culture extracts was based on the use of externalstandards. For quantitative analyses, three ions, m/z 205 (MH⁺), m/z109, and m/z 95, were measured by selected ion monitoring in the CImode. The size of the m/z 109 fragment was used to determine the amountof trichodiene in the culture extracts. The response of the m/z 109signal at the proper retention time for trichodiene was linear acrossthe range from 1 ng to 2 μg. Typical coefficients of variation forreplicate analyses were between 8 and 15%.

EXAMPLE 6

Isolation of Trichothecenes

A 4-L culture of the mutant NRRL 18339 was extracted twice with an equalvolume of ethyl acetate. The combined extracts were evaporated todryness and resuspended in 20 ml of ethyl acetate. This extract wasseparated on a preparative silica cartridge using a Waters prepmasterliquid chromatograph. The column was eluted with CH₂ Cl₂ /methanol(98/2).Eighteen 500-ml fractions were collected. Then the column waswashed with 500 ml of methanol. The separation was monitored by TLC.Fractions 7 and 8, which contained Compound 1 (below), were combined foradditional purification. Compound 2 (below) was found by TLC to be inthe methanol (fraction 19). This fraction was reapplied to the silicacartridge and eluted with CH₂ Cl₂ /methanol (95/5). Fourteen 500-mlfractions were collected and then the column was washed with 500 ml ofmethanol. Compound 2 was shown to be in fractions 4 and 5, which werecombined for further purification. ##STR1##

The combined fractions containing Compound 1 were purified further byJPLC on a Whatman PAC column. The separation was developed on ananalytical column (20 cm×4.6 mm) and then preparative separations weremade using the "Magnum-9" column (50 cm ×9.5 mm). The eluting componentswere detected with a differential refractometer. Compound 1, 37 mg, wasisolated from repeated injections of portions of the fraction containingit. The solvent was hexane/ethyl acetate (60/40). It eluted as a peakwith a retention of about 3 column volumes. Compound 2, 26 mg, wasisolated from fractions containing it. Hexane/ethyl acetate (25/75) wasthe solvent.

EXAMPLE 7

Large Scale Production of Trichodiene

Each of three flasks containing autoclaved rice (333 g) was inoculatedwith an 11-ml aliquot of a 48-hr-old liquid shaker culture which hadbeen inoculated with 5×10⁴ conidia and incubated for 2 weeks at 28° and200 rpm. The entire culture was extracted three times with three volumesof ethyl acetate.

The combined ethyl acetate extracts were evaporated to yield an oilyresidue which was applied to a 3-in pad of silica gel in a scinteredglass funnel (7.0 cm×9.5 cm id). The trichodiene was eluted from the gelwith hexane and was present in the first 700 ml collected. A total of 10g of trichodiene was thus obtained. ##STR2##

It is understood that the foregoing detailed description is given merelyby way of illustration and that modification and variations may be madetherein without departing from the spirit and scope of the invention.

We claim:
 1. A Fusarium sporotrichioides mutant strain having theidentifying characteristics of ARS Culture Collection Accession No.18339; said strain being capable of producing dideacetylcalonectrin anddeacetylcalonectrin in amounts greater than 100 μm/L.